1. Field of the Invention
This invention relates to new and useful improvements in methods for measuring the fracture toughness of a ductile or brittle specimen material.
2. Prior Art
Plane strain fracture toughness measurement techniques making use of the principles of linear elastic fracture mechanics have been developed over about the last two decades. Such methods have generally involved introduction of a crack into an appropriate specimen configuration, a measurement of the crack position, a measurement of the load necessary to cause the crack to advance, and a measurement or a calculation of the rate of change of the specimen's compliance with respect to the distance of crack propagation.
Drawbacks to present fracture toughness measurement techniques that are overcome by the present invention include: that it is usually very difficult to introduce a real crack into a specimen of brittle material; such experiments and data collection therefrom have, heretofore, been complicated; and such experiments have required that both the crack position and the crack-advancing load must be measured during the experiment.
Reference is hereby made to a patent entitled "Fracture Toughness Test Method," by the present inventor, Lynn M. Barker, which patent is assigned to and filed by Reed Tool Company, Houston, Tex. This patent deals with an Improved Method for Testing Fracture Toughness of Tungsten Carbide Samples. Unlike the present invention, the method taught by this process involves comparing the results of identical tests run on two specimens of identical size and shape but of different materials, and hence only comparative results are obrtained from this procedure. The present invention, unlike the above cited application for patent, produces a measurement of the actual megapascal times square root meter, or psi times square root inch, value of the critical stress intensity factor (fracture toughness) of a specimen material. While the present invention, like the above cited application for patent, can be used for testing of tungsten carbide samples, it should be understood that the present invention is not limited to tungsten carbide samples only, and encompasses a determination of the fracture toughness of a specimen from consideration of: the peak or critical load executed on the sample, the sample geometry and size, and Poisson's ratio, for the specimen only. Further, the method of the present invention is suitable for testing of many other brittle and ductile materials. The present invention is therefore not limited, as is the above cited patent application, to brittle materials only and does not require data comparison with a standard specimen.
Testing procedures and practices involving testing of crack growth stability in ductile and brittle materials have been recorded in works like that of H. G. Tattersall and G. Tappin, "The Work of Fracture and its Measurement in Metals, Ceramics, and Other Materials." J. of Mater. Sci 1, 296 (1966). As reported therein, the procedures have involved a segment of a rod specimen, the specimen having a notch cut therein leaving a triangular cross-sectional remainder across a central crack plane such that a crack started at the apex of that triangular remainder will continue to widen uniformly as it is propagated through the specimen with incremental increases in the bending stresses applied to the sample. The results of such testing enabled the authors to calculate fracture toughness by integrating the load-deflection curve for the material. That work, however, did not involve a determination of the fracture toughness of a specimen from consideration of the peak or critical load exerted on the sample; the sample geometry; the sample size; and Poisson's ratio, for the specimen only.
A number of other investigations have been undertaken involving stable crack growth. Some such references include: S. Mostovoy, R. P. Crosley, and E. J. Ripling, "Use of Crack-Line Load Specimens for Measuring Plane-Strain Fracture Toughness," J. Mater. 2, 66 (1967); H. L. Marcus and G. C. Sih, "A Crackline-Loaded Edge-Crack Stress Corrosion Specimen," Eng. Frac. Mech. 3, 453 (1971); J. P. Gallagher, "Experimentally Determined Stress Intensity Factors for Several Contoured Double Cantilever Beam Specimens," Eng. Frac. Mech. 3, 27 (1971); J. A. Kies and A. B. J. Clark, in Proceedings of the Second International Conference on Fracture, paper 41, Brighton (1969); A. G. Evans, "A Method for EValuating the Time-Dependent Failure Characteristics of Brittle Materials -- And its Application to Polycrystaline Alumina," J. Mater. Sci. 7, 1137 (1972); R. C. Clifton, E. R. Simonson, A. H. Jones and S. J. Green, "Determination ofthe Critical Stress Intensity Factor K.sub.IC from Internally-Pressurized Thick-Walled Vessels," published in Experimental Mechanics. None of these references or any reference known to the inventor, has, however, involved, as does the present invention, the measurement of fracture toughness from considerations of only the peak (critical) load, the specimen geometry, the specimen size, and Poisson's ratio, for the specimen material.
Within the knowledge of the inventor, the method and technique of the present invention is unlike any heretofore known, and is therefore, believed to be both novel and unique.